加密是 保证数据安全的手段之一。加密是将纯文本数据转换为难以理解的密文；解密是将密文转换回纯文本。
数据的加解密属于密码学的范畴。通常，加密和解密都需要使用一些秘密信息，这些秘密信息叫做密钥，将纯文本转为密文或者转回的时候都要用到这些密钥。
对称加密指的是发送者和接收者共用同一个密钥的加解密方法。
非对称加密(又称公钥加密)指的是需要一个私有密钥一个公开密钥，两个不同的密钥的加解密体系。尽管不同，这个密钥对的这两个部分在算法上是有关联的。一个密钥将纯文本加密，另一个将密文解密。没有一个密钥能够把加密和加密的功能全部自己完成。公钥，或者用于加密数据的密钥，可以被自由分发。
RSA 是基于大整数分解的公钥加密的算法之一。RSA 代表了 Ron Rivest、Adi Shamir 和 Leonard Adleman 三人，RSA 就是他们一起提出的。
以下示例就展示了如何在 Java 中使用 RSA 算法对信息进行加解密。
java.security.KeyPairGenerator 类的实例用于产生一个 RSA 算法的公钥和私钥对，之后将其保存到文件。
javax.crypto.Cipher 类的实例用于使用上面产生的密钥对对信息进行加解密。 

import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.security.KeyPair;
import java.security.KeyPairGenerator;
import java.security.NoSuchAlgorithmException;
import java.security.PrivateKey;
import java.security.PublicKey;

import javax.crypto.Cipher;

/**
 * @author JavaDigest
 * 
 */
public class EncryptionUtil {

  /**
   * String to hold name of the encryption algorithm.
   */
  public static final String ALGORITHM = "RSA";

  /**
   * String to hold the name of the private key file.
   */
  public static final String PRIVATE_KEY_FILE = "C:/keys/private.key";

  /**
   * String to hold name of the public key file.
   */
  public static final String PUBLIC_KEY_FILE = "C:/keys/public.key";

  /**
   * Generate key which contains a pair of private and public key using 1024
   * bytes. Store the set of keys in Prvate.key and Public.key files.
   * 
   * @throws NoSuchAlgorithmException
   * @throws IOException
   * @throws FileNotFoundException
   */
  public static void generateKey() {
    try {
      final KeyPairGenerator keyGen = KeyPairGenerator.getInstance(ALGORITHM);
      keyGen.initialize(1024);
      final KeyPair key = keyGen.generateKeyPair();

      File privateKeyFile = new File(PRIVATE_KEY_FILE);
      File publicKeyFile = new File(PUBLIC_KEY_FILE);

      // Create files to store public and private key
      if (privateKeyFile.getParentFile() != null) {
        privateKeyFile.getParentFile().mkdirs();
      }
      privateKeyFile.createNewFile();

      if (publicKeyFile.getParentFile() != null) {
        publicKeyFile.getParentFile().mkdirs();
      }
      publicKeyFile.createNewFile();

      // Saving the Public key in a file
      ObjectOutputStream publicKeyOS = new ObjectOutputStream(
          new FileOutputStream(publicKeyFile));
      publicKeyOS.writeObject(key.getPublic());
      publicKeyOS.close();

      // Saving the Private key in a file
      ObjectOutputStream privateKeyOS = new ObjectOutputStream(
          new FileOutputStream(privateKeyFile));
      privateKeyOS.writeObject(key.getPrivate());
      privateKeyOS.close();
    } catch (Exception e) {
      e.printStackTrace();
    }

  }

  /**
   * The method checks if the pair of public and private key has been generated.
   * 
   * @return flag indicating if the pair of keys were generated.
   */
  public static boolean areKeysPresent() {

    File privateKey = new File(PRIVATE_KEY_FILE);
    File publicKey = new File(PUBLIC_KEY_FILE);

    if (privateKey.exists() && publicKey.exists()) {
      return true;
    }
    return false;
  }

  /**
   * Encrypt the plain text using public key.
   * 
   * @param text
   *          : original plain text
   * @param key
   *          :The public key
   * @return Encrypted text
   * @throws java.lang.Exception
   */
  public static byte[] encrypt(String text, PublicKey key) {
    byte[] cipherText = null;
    try {
      // get an RSA cipher object and print the provider
      final Cipher cipher = Cipher.getInstance(ALGORITHM);
      // encrypt the plain text using the public key
      cipher.init(Cipher.ENCRYPT_MODE, key);
      cipherText = cipher.doFinal(text.getBytes());
    } catch (Exception e) {
      e.printStackTrace();
    }
    return cipherText;
  }

  /**
   * Decrypt text using private key.
   * 
   * @param text
   *          :encrypted text
   * @param key
   *          :The private key
   * @return plain text
   * @throws java.lang.Exception
   */
  public static String decrypt(byte[] text, PrivateKey key) {
    byte[] dectyptedText = null;
    try {
      // get an RSA cipher object and print the provider
      final Cipher cipher = Cipher.getInstance(ALGORITHM);

      // decrypt the text using the private key
      cipher.init(Cipher.DECRYPT_MODE, key);
      dectyptedText = cipher.doFinal(text);

    } catch (Exception ex) {
      ex.printStackTrace();
    }

    return new String(dectyptedText);
  }

  /**
   * Test the EncryptionUtil
   */
  public static void main(String[] args) {

    try {

      // Check if the pair of keys are present else generate those.
      if (!areKeysPresent()) {
        // Method generates a pair of keys using the RSA algorithm and stores it
        // in their respective files
        generateKey();
      }

      final String originalText = "Text to be encrypted ";
      ObjectInputStream inputStream = null;

      // Encrypt the string using the public key
      inputStream = new ObjectInputStream(new FileInputStream(PUBLIC_KEY_FILE));
      final PublicKey publicKey = (PublicKey) inputStream.readObject();
      final byte[] cipherText = encrypt(originalText, publicKey);

      // Decrypt the cipher text using the private key.
      inputStream = new ObjectInputStream(new FileInputStream(PRIVATE_KEY_FILE));
      final PrivateKey privateKey = (PrivateKey) inputStream.readObject();
      final String plainText = decrypt(cipherText, privateKey);

      // Printing the Original, Encrypted and Decrypted Text
      System.out.println("Original: " + originalText);
      System.out.println("Encrypted: " +cipherText.toString());
      System.out.println("Decrypted: " + plainText);

    } catch (Exception e) {
      e.printStackTrace();
    }
  }
}